In a cryogenic air separation unit, the carbon dioxide and water vapor must be removed from the compressed air prior to entering the cryogenic portion of the plant. The carbon dioxide and water vapor can be removed by a variety of methods, but is typically removed in an adsorption process. This adsorption process may be either temperature swing or pressure swing. Pressure swing adsorption (PSA) systems use either two or three bed cycles. A typical two bed adsorption cycle is shown below where "Depress" stands for depressurization and "Repress" stands for repressurization: ##STR1##
The primary operational problem for a two bed PSA system is the requirement that the off-line bed be repressurized from the clean air stream prior to the bed switch. This causes a temporary decrease in the air flow to the air separation unit of typically between 5-15% of the main air compressor flow for a period lasting typically between 3-5 minutes.
In most plants, the main air compressor does not have sufficient capacity to supply the extra air required to repressurize the off-line bed without impacting the flow of air to the air separation unit. For a PSA system with cycle times of 1 minute for depressurization, 10 minutes for purging, 3 minutes for repressurization and a peak repressurization flow of 10% of the air flow, the air flow to the air separation unit as a function of time will be as shown in FIG. 1. As can be seen in FIG. 1, a portion of the air flow to the air separation unit is diverted every eleven minutes for a period lasting three minutes and, when diverted, constitutes 10% of the total impurity-deleted air feed at the beginning of such three minute period, gradually falling to 0% at the end of such three minute period.
The decrease in air flow to the air separation unit causes a disturbance to the purities within the lower pressure column and may cause product purities to violate their respective purity specifications. Furthermore, for plants with an argon sidearm column for argon purification, there is a significant risk of increased nitrogen in the feed to the sidearm column which can indirectly cause a trip of that column.
Air separation units with early implementations of two bed PSA systems were run at reduced recovery. For example in a 1995 AICHE paper by Megan et al. of Praxair, Inc. entitled "Dynamic Study Of Air Flow Disturbances On A cryogenic Air Separation Plant, the impact of two bed disturbances was discussed and was solved by running the air separation unit with higher purity product streams than required, to allow for dips in purity that occurred because of a repressurization step. This reduced the recovery of the air separation unit along with increasing the power costs per unit of production.
The three bed PSA cycle was specifically developed to minimize the repressurization disturbance by continually repressurizing one of the three beds. This has been patented by Kumar and assigned to The BOC Group, Inc. as U.S. Pat. No. 5,560,763. The operational benefit of the third bed comes with a significant capital cost however. The penalty associated with a third bed and the associated piping and valve costs can be as much as $500,000.00 on a large plant.
U.S. Pat. No. 5,406,800 by Bonaquist and assigned to Praxair Technology, Inc. teaches using the high pressure and/or other column sumps to maintain column purities in response to load following where changes in the product flow from the air separation unit are desired. Contrast this with the goal of the present invention where the product flow and the product purities from the air separation unit are desired to be constant.